In many cases, a heat developable light-sensitive material contains a base or a base precursor in the light-sensitive material to accelerate heat development. From a viewpoint of storage stability of the light-sensitive material, use of a base precursor releasing a basic substance by thermal decomposition is more preferred.
Typical examples of the base precursor are described in British Pat. No. 998,949. Preferred base precursor is a salt of a carboxylic acid and an organic base. Examples of useful carboxylic acid are trichloroacetic acid and trifluoroacetic acid, and examples of useful base are guanidine, piperidine, morpholine, p-toluidine, and 2-picoline. Particularly useful base precursor is guanidine trichloroacetate as described in U.S. Pat. No. 3,220,846. Further, aldoneamides described in Japanese Patent Application (OPI) No. 22625/75 (the term "OPI" as used herein means a "published unexamined Japanese patent application") decompose at high temperatures to release a base, and are preferably used.
Of the base precursors described above, water-soluble base precursors, however, have a disadvantage such that they are easily changeable on reacting with other components contained in coating materials. Furthermore, since those water-soluble base precursors are added in the form of an aqueous solution, those are uniformly present in the coating and are readily affected by air or moisture. Hence, the water-soluble base precursors are decomposed under the action of air or moisture to change photographic characteristics of the light-sensitive material, thereby deteriorating the storage stability of the light-sensitive material.
Water-insoluble base precursors have heretofore been used in the manner such that these are first dissolved in an organic solvent which is compatible with water, such as methanol, ethanol, acetone, or dimethylformamide, and then the resulting solution is added to an emulsion layer and/or its adjacent layer of the light-sensitive material. This is an industrially convenient method to introduce a water-insoluble additive into the light-sensitive material. In the method, however, the amount of the solvent which can be introduced into the light-sensitive material is limited. No serious problem arises when the amount of the additive added is small but in the case of the base precursor which must be added in a large amount, the amount of organic solvent which is required to dissolve therein the base precursor often exceeds the upper limit. Furthermore, many base precursors are sparingly soluble in such organic solvent which is compatible with water and those are difficult to add to the light-sensitive material.